A friend and I were discussing the recently completed Spartacus series. The series was still high quality and producing great ratings for Starz, but the producers decided to end it this year, rather than drag out the rebellion. Although fans were disappointed there wasn't another season, I think it was very smart for them to end it after 3 seasons (4 if you count the prequel). Even before I knew they were ending it this year, I felt the story arc consisted of Spartacus escaping, Spartacus killing Glaber, and Spartacus' army finally losing to Crassus. Those 3 events were great climaxes for their seasons and I think the show benefited from having their seasons build up to those 3 historical points.
In contrast, I've seen many great shows with interesting concepts have great first seasons but then sort of fizzle out because the writers are trying to drag the story too long, in order to make more money for the networks. That was initially the problem with lost which had a great first season, then had some sucky seasons until they decided when they would end it. I'm sort of seeing it now with Revenge as they now have this whole mess with the initiative. And it was definitely a problem with Desperate Housewives after the first season when they solved the mystery but then had to keep adding more stuff.
So I'm wondering if the American model where TV shows last as long as they can is really better than the model in other countries where shows are only planned for one or two seasons, and the show runners definitely know how it is going to end. Thoughts?